The invention relates to an analogue scrambling system in which the processing of the speech signal accomplished in digital signal processors comprises the following operations: filtering, sampling and digitizing in an analogue-to-digital converter, processing by the analysis filter bank transforming the signal sampled at the rate f.sub.e into N sub-band signals sampled at f.sub.e /N and transferred in a permuted order to the synthesis filter bank which carries out the calculations of the scrambled signal sampled at the rate f.sub.e to which is digitally added the synchronizing wave sin(2.pi.nTf.sub.e /4), wherein T is the duration of the sampling cycle, while the scrambled digital signal obtained thus is converted into an analogue signal, filtered and transmitted through an analogue channel to the unscrambler where a preprocessor effects the synchronizing of the sampling, the compensation of the said synchronizing wave and the equalizing of the scrambled signal and where the processes that have been accomplished are identical with those accomplished at the scrambler but for the fact that the said permuted order of the N sub-band signals is reversed.
Such a system is utilized for ensuring the confidentiality of communication on a radio channel. The scrambling systems can generally be subdivided into two large families: the digital scrambling systems and the analogue scrambling systems.
The former systems do not require a digitizing and encoding of the speech signal, as the resulting binary output is encrypted by means of a pseudo-random sequence.
The degree of security obtained is potentially the highest possible, that is to say that the message cannot be decoded without knowing the key. The problem we are facing is the transmission of the signal on a standard radio channel of 3 kHz pass-band. After all, such a transmission can only take place with the aid of modems operating at 2400 or 4800 bits/s requiring the encoding of the speech signal to work at these outputs for which at best only the intelligibility of the message can be ensured. In addition, such systems of which the implementation is rather complex, can only be suitable for use in networks or connections whose subscribers are special operators (army, police, . . . ) who can accept to converse with a signal quality which is strongly degraded.
The analogue scrambling systems are distinguished from the known systems in that the waveform of the transmitted signal directly originates in the transformations of the waveform of the original speech signal. The transformations can be effected in the time domain, the frequency domain or in the two simultaneously according to the degree of confidentiality wanted. However, it should be observed that absolute security cannot be attained with this type of systems. On the other hand, they have the advantage that they can be realized in a simpler manner and offer a quality of the restored signal which is better than in the digital systems.
Historically, the first analogue scramblers were based on spectral transformations of the invention type, displacement or band permutation type. By utilizing analogue techniques, the realized scrambling showed drawbacks of which the main ones were a relatively high residual intelligibility as well as a robustness at very moderate codebreaking. For example, the band permutation technique was limited to 5 sub-bands which did not permit effective scrambling of the signal. With the appearance of memories and microprocessors, the techniques using temporary transformation have come into existence. They are based on the principle of signal block permutation of 10 to 20 ms. Thus, the distribution of the signal strength as a function of time differs from that of the original speech signal, whereas this distribution is the same in the spectral scramblers. On the other hand, the waveform of the phonemes permuted in time remains unchanged. This constitutes impairment to the direct locking of the scrambled signal with a view to restoring the order of the segments of the permuted speech signal. In addition, for ensuring the lowest possible residual intelligibility, the delays used can become very important (several hundred ms), as they can give rise to a constraint in the communication.
Based on the two techniques described before, relatively effective scramblers can be designed by cascading the temporary and spectral transformations. Yet, with the appearance of the digital signal processors very effective scrambling techniques can be designed which are in fact based on the concepts of the first scramblers and, more specifically, on frequency band permutation. By using digital techniques it is possible to do away with the problems of drift affecting the modulators, demodulators and filters used in an analogue system. Thus, it can be considered to divide a signal into a large number of frequency band signals, thereby improving the quality of the scrambling. Furthermore, the fact that quadrature mirror filters (QMF) banks can almost perfectly restore the original signal, allows to consider a very effective spectral scrambling system.
We now discuss the state of the art as regards the analogue scrambling systems with digital processing utilizing spectral permutation and of which the processing takes place in a digital form. They can be subdivided into three types:
systems coefficient permutation using the Discrete Fourier Transform,
systems of band permutation using filter banks which do not ensure a perfect restoration of the band,
systems of band permutation using filter banks denoted QMF or pseudo-QMF.
The systems of the first type have the characteristic feature that this signal is not modified at all when, without affecting the permutation, a transform is followed by an inverse transform. As a matter of fact, it is known that DFT.sup.-1 (DFT)=Identity. Nevertheless, the filter bank realized as such is of a very poor quality in that on the one hand the filtering function is of the type sinx/x, and on the other hand, the recoveries between the filters are very important. Thus, the verification of the encrypted signal band is not very easy and, in addition, the residual intelligibility of the encrypted message suffers from the "softness" of the filters.
These problems have been resolved with the aid of very selective filter banks which allow to dispense with synchronization. This feature may seen attractive but in fact is a drawback in that the whole transmitted message is permuted in the same way. Moreover, the filters utilized do not have the characteristic feature of having the unitary composite analysis-synthesis response and thus the quality of the reproduced signal is mediocre.
The last type of systems incorporates the advantages of the two first systems in so far as they utilize QMF or pseudo-QMF filter banks permitting a relatively selective time-frequency band division which is substantially perfect. The U.S. Pat. No. 4,551,580 discloses a scrambling system of this type and of the same kind as that described in the preamble.
In this system the QMF filter banks are utilized for dividing the signal into 5 sub-bands, with 25 consecutive samples of each sub-band constituing a block. Thus the permutation affects all 125 samples of the 5 blocks. This permutation is fixed by the choice of the key. Although the system is complex, this being fixed is a drawback.
A further characteristic features of this system is the use of an equalizer which only compensates for the channel phase while assuming that the module is unitary. This implies that the system can only be used for telephone lines and not for a mobile radio link. In addition, the principle of sending a Dirac pulse for measuring the pulselike response would be highly impracticable on a radio link.
Speech scrambling systems based on dynamic band permutation have already been obtained by analogue processing. The object of the invention is to propose a system always utilizing dynamic permutation, but which system is obtained on the basis of digital processing substantially perfectly realized in an easy way by analysis and synthesis filter banks with the aid of pseudo-QMF filters and a dividing of the signal into a large number of sub-band signals which can be permuted at a very high rate.